Ultraluminous Distant Ellipticals Revealed in ISO + SPITZER Surveys

The galaxy populations discovered in the deepest UV-optical and infrared surveys appear so different that the basic physics of galaxy evolution is still debated. In Rocca-Volmerange et al., 2007 (hereafter RVLS2007), we proposed an interpretation of the deepest mid-IR faint galaxy counts in accordance with the already-published UV-optical NIR count analysis (Fioc & Rocca-Volmerange, 1999).

The main constraint is to reproduce the galaxy number excess observed at 12 μm in the ISO-ESO-Sculptor Survey (Seymour et al., 2007) as at
15 μm and 24 μm in other ISO and SPITZER surveys. The originality of the analysis is to follow continuously the evolution of stellar and dust emissions from UV to infrared as a function of
galaxy type. We use the new version of our evolutionary code PÉGASE.3, extended to dust emission. Evolving UV to IR SEDs allow to compute robust k- and e- (evolution) corrections for all z and types to predict high z luminosities.

We find that the fractions by galaxy types from the optical counts are all detected in the mid-IR counts, with
the exception of a minor fraction of ellipticals (< 10% of all galaxies) which appear ultra-bright and dusty. The
differential brigthness excess (-2.5 mag at 12 μm, -5 at 24 μm) confirms the presence of dust, while the bump is explained by strong redshifted stellar emission of ellipticals (k+e corrections). The model is valid at 12 μm, 15 μm and 24 μm. It does not need any redshift-dependent starbursts. No number density evolution is included in our models. These ULIRGs are massive evolved galaxies formed at early epochs and likely hosts of AGNs. They will be essential targets for the future telescopes SPICA, HERSCHEL
and ALMA.